In the paper making industry where paper is being produced at a high rate from a paper making machine, for quality and feedback control the paper is scanned crosswise by a moving head containing a number of sensors to determine parameters such as basis weight, moisture and gloss. The final value of gloss is a rather arbitrary number determined by standards in the paper making industry; namely, TAPPI standard T 480 om-90 which involves projecting onto the paper surface an incident beam of light at a particular angle (for example, 15.degree.), detecting the reflected beam and measuring its intensity. To calibrate the above TAPPI standard a polished black glass standard is used. Then an intermediate standard which is calibrated against that may be a polished ceramic tile. Some gloss sensors actually mechanically carry such a tile in a moving measuring head and lower the tile into the light beam to calibrate the instrument. This, of course, is mechanically complicated and there are some problems of environmental conditions such as heat, dirt and also accurate positioning.
Another gloss technique of Valmet Automation of Canada provides a separate reference beam apart from the incident measuring beam. Here there are two separate light sources and detectors; moreover, the light source is a different type than the standard source defined by the above TAPPI standards. Thus, the correlation to the industry standard is suspect.
Finally the moving paper sheet inherently tilts or produces waves so that the surface moves to, in some cases, during the measuring process cause the reflected beam to miss the detector entirely. Also there are parallel shifts.
All of the foregoing implies a reliability of gloss measurement much less than desired.